ON THE VERTEBRATE SKELETON. 315 



animals. In the fish, in fact, — by reason of the parietal parapophyses (3, s) 

 being subject to the same variation in their relative position to the other 

 elements, which has been illustrated in respect of the neural spine in the 

 epencephalic arch of the dog and sheep, — the mesencephalic arch is com- 

 posed of seven pieces, or, including the interposed supraoccipital, of not less 

 than eight bones. Yet even here we clearly and easily trace the kind and 

 degree of modification to which the fundamental plan of the neural arch 

 has been subject. The archetype is nowise obliterated : the general homo- 

 logies of the modified elements are not less recognisable than their special 

 homologies. The centrum and neurapophyses are the steadiest elements : 

 the spine is not only subject to great diversity of size and shape, but to some 

 variety of position, and, moreover, to be either single or bifid : the parapo- 

 physes have less range of variety in point of dimensions, but may be more 

 or less interposed between spine and neurapophyses, or may become con- 

 fluent with either element. Thus the epencephalic arch of the crocodile 

 (fig. 18) differs essentially, in a Cuvierian sense, from that of the tortoise or the 

 fish (fig. 1), because it is composed of four pieces in the first and of six 

 pieces in the latter ; the difference of composition merely depending, how- 

 ever, on the more exterior position and connation of the parapophyses, 4, 4, in 

 the crocodile. 



The independency of the parietal and frontal bones is next urged by 

 Cuvier as militating against the idea that they complete a vertebral arch 

 formed respectively by the alisphenoids and orbitosphenoids as the piers or 

 haunches : and the more so, inasmuch as they are separated from those bones 

 in some animals by the intercalation of the squamosals*. By parity of reason 

 we must reject the general homology of the neural arch and spine of the 

 atlas in the Ephippus and some other fishes, because that part of the verte- 

 bra is not only distinct, but uplifted and removed from the piers or base of 

 the arch by the intercalation of the articular processes of the neural arches 

 of the occiput and axis. According to Cuvier such separated atlantal arch 

 must be regarded as a new bone, and the centrum ought therefore equally 

 to be viewed as ' une piece particuliere qui a une destination particuliere ' : 

 but the general homology of vertebral elements may be determined not only 

 by their relations to their own segment, but by those which they maintain 

 with their less modified homotypes in contiguous segments. 



The centrum of the atlas in the Ephippus directly sustains other neur- 

 apophyses than its own, and so far has a new or particular function ; but, 

 since it continues to unite the centrum of the axis with that of the occiput, 

 we still regard it as their homotype, and as standing in the relation of the 

 centrum to its uplifted and shifted neurapophyses. So, likewise, although 

 these elements now aid in strengthening the joint between the zygapophyses 

 of the neural arches of the occiput and axis, and thus perform a new and 

 very peculiar function, their relation to these and other neural arches in the 

 series of vertebrae renders it impossible to overlook the serial homology of 

 the separated ' laminae ' of the atlas and that of its spine with the other and 

 larger vertebral laminae and spines. 



* " Dans tous les cas, on ne pourrait regarder cette vertebre comme annulaire, ni supposer 

 que les parietaux en forment le complement ; d'une part, ce serait une composition differente 

 de celle des autres vertebres, puisque l'anneau serait forme de cinque pieces et meme de 

 six, en comptant l'inter-parietal ; de 1' autre, il arrive dans plusieurs animaux que les ailes 

 temporales du spheno'ide n'atteignent pas au parietal, parceque le temporal va toucher au 

 dessus d'elles, soit an frontal soit au spheno'ide anterieur. Ainsi les parietaux sont des 

 pieces independantes du spheno'ide posterieur, des pieces particulieres qui ont une desti- 

 nation particuliere, celle de servir de bouclier a la partie moyenne et posterieure des hemi- 

 spheres, tout comme les grandes ailes ont celle de servir de support aux lobes moyens dans 

 lesquels ces hemispheres se terminent vers le bas." — I. c. p. 713. 



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